514

DYEING AND CALICO PRINTING.

however, visible in the cut, being placed in front of and before n ); m servesfor the introduction of pure water, n and o for running off the decoction madeof the dye-wood under pressure of the steam, but if the steam is turned offthe liquor is run off by q, a small air admission tap being provided at p.

The evaporation of the liquors obtained from the exhaustion of the woodswith water is effected in various ways ; sometimes vacuum-pans are used, butthey do not answer the purpose very well for extracts which have to be con-centrated above a specific gravity of i'i34, on account chiefly of the difficultyexperienced by the bubbles of aqueous vapour in escaping from a mass ofviscous liquid; the evaporation is therefore . most frequently carried on bymeans of double bottomed shallow pans heated by steam.

In order to make extract of quercitron, this material, which is importedready pulverised, has to be boiled for a length of time with water, and the

Fig. 23.

liquid to be separated from the dregs by filtration, the residue being pressed,and the clear liquid is then evaporated to dryness.

The extracts of dye-woods contain, in addition to the colouring matter, allthe soluble salts contained in the wood, also the nitrogenous matter, theglucosides, and other vegetable matters soluble in water, which it would bevery useful to remove if it could be done efficiently and without too greatexpense, since most of these substances impair the beauty and brightness of thedyes made with these extracts, and, moreover, add to the cost of transport,as well as render the extract liable to become mouldy. A great desideratumalso is the obtaining of inexpensive and suitable solvents for the removal ofthe colouring matters from the woods in a purer state. It has been proposed